High Intensity Discharge (HID) lamps are primarily used for large area illuminations. Unlike an incandescent bulb, a ballast device is necessary to ignite and operate an HID lamp. There are three primary functions of a ballast device, namely: 1) lamp ignition, 2) lamp power control, and 3) control of AC line transients and power quality. Most commonly used HID lamps are high-pressure sodium, ceramic, and pulse start metal halide lamps. All of these lamps require very high voltage for their ignition. Recently, electronic ballasts have been introduced to replace the old magnetic ballasts for higher operating efficiency and to improve lamp life and lumen maintenance by incorporating controlled ignition and lamp power regulation schemes.
There are two types of electronic ballasts: ballasts that operate HID lamps with a frequency that is higher than 20 kHz and ballasts that operate HID lamps with a frequency that is lower than 800 Hz. The primary advantage of low frequency operated electronic ballast is that it doesn't give rise to destructive acoustic resonance in the HID lamp. In particular, lower wattage ceramic metal halide electronic ballasts with a built-in integral igniter are designed to operate HID lamps at a low frequency. Although the lamp power regulation is important and offers other benefits, it is the proper lamp ignition, controlled glow-to-arc, and arc stabilizations that minimize electrode sputtering and ultimately determine the useful life of an HID lamp. Various schemes were proposed in the past to achieve these characteristics, but they are either too complex or lack completeness.
The starting and operating of an HID lamp involves five stages: two breakdowns, cold cathode, glow-to-arc transition, and thermionic arc. Prior to the initial breakdown, the HID lamp offers very high impedance. Upon application of a high voltage across an HID lamp, the induced electric field generates seed electrons which cause a breakdown. At the instant of the breakdown, the lamp impedance drops drastically but almost immediately increases to a higher value as the HID lamp enters into a glow discharge phase.
Further, following the breakdown, the HID lamp may enter into either a cold cathode or glow discharge phase, depending on whether or not mercury in the HID lamp has condensed on the electrodes. In order to sustain a glow discharge, the voltage across the lamp electrodes attains a higher value, but it is much lower than the breakdown voltage. The primary source of electrons during this period is secondary emission from the lamp electrodes by ion bombardment, which causes both sputtering and heating of the electrodes. The heating of the lamp electrodes raises the cathode temperature sufficiently high for thermionic emission to occur, which is the beginning of the glow-to-arc transition. In order to sustain thermionic emission, the ballast must supply sufficient current into the HID lamp for establishing a conductive arc between the lamp electrodes. Once the arc phase (takeover) begins, it takes between a few hundred seconds to a few minutes before it fully develops.
The starting of an HID lamp and also the associated sputtering of the electrodes affect the lamp life and the lumen maintenance. In order to minimize electrode sputtering during the glow and initial arc phase, balanced and controlled transitions are required, which are difficult to incorporate in the design of magnetic ballast.
Low frequency electronic ballast operates HID lamps with a frequency that is usually less than 300 Hz. The lamp operating waveform is an alternating square wave, which is typically created by a conventional full bridge inverter circuit. The square wave operation of the HID lamp yields: a) low lamp current crest factor, b) unity lamp power factor, and c) fast polarity transition, which results in higher efficiency, lower lamp voltage, and flicker free constant light output.
Low frequency electronic ballasts also primarily utilize two ignition schemes. These are: a) superimposing a breakdown voltage on top of the low frequency square wave (which is similar to the ignition scheme of magnetic ballasts) and b) high frequency resonant generated ignition pulses. As stated earlier, it is the quality of the ignition and transitions that primarily determine useful lamp life and light output. Therefore, low frequency electronic ballasts that utilize magnetic ballast ignition schemes provide certain improvements but not all that are desired. Low frequency electronic ballasts that utilize a controlled high frequency resonant scheme for lamp ignitions and then switch to a low frequency for normal operation can provide additional improvements.
However, none of the electronic ballasts provide a complete electronic ballast architecture that ensures proper lamp ignition and operation, since they typically address a limited number of aspects of lamp ignition and operation.